1. Field of the Invention
The present invention relates to a method of disordering quantum well heterostructures, a method of tuning the wavelength range of quantum well optoelectronic devices, and structures or devices produced by these methods.
2. Description of the Related Art
The wavelength range of quantum well optoelectronic devices can be tuned by disordering (or ‘interdiffusing’ or ‘intermixing’) the quantum well boundaries. Although this can be achieved by various types of processing, ion irradiation or implantation is a particularly attractive method because it is accurate, reproducible, and easily confined to arbitrary lateral geometries. Moreover, the range of depths disordered by irradiation may be tailored by selecting appropriate ion beam energies. FIG. 1 shows a prior art optoelectronic device 1 including a quantum well heterostructure 2. This device 1 can be fabricated by depositing alternating thin layers of two semiconductors with different energy band structures on top of a bottom contact layer 24 grown on a substrate 4, and then depositing a top contact layer 26. The heterostructure 2 contains layers 6 to 12 of a first semiconductor and layers 13 to 18 of a second semiconductor. The heterostructure 2 is defined as the region containing layers 6 to 18 located between the top interface 5 and the lowest interface 19. The heterostructure layers 6 to 18 may be grown by techniques such as molecular beam epitaxy (MBE) or chemical vapour deposition (CVD). If the layers 6 to 12 with the smaller band gap are sufficiently thin, they can form quantum wells which trap charge carriers and constrain their energies to a series of bound states according to the laws of quantum mechanics. Such a heterostructure can be used to fabricate optoelectronic devices with operating wavelengths determined by the energy states of the quantum well, which depend upon the band structures of the semiconductors and the thicknesses and compositions of the well layers 6 to 12 and the barrier layers 13 to 18. However, the wavelength range of an existing heterostructure may be changed or ‘tuned’ by disordering or ‘intermixing’ the semiconductor layers to some extent. This changes the well properties and therefore the energy levels of the well and hence the operating wavelength range. Intermixing is effected by supplying point defects to the quantum well interfaces. Ion implantation is a useful method for generating point defects in a controlled manner, and can be used for quantum well intermixing. It is especially useful because lithographic techniques may be used to mask selected areas to restrict intermixing to the exposed areas. This allows different optoelectronic devices on the same wafer to operate in different wavelength regions.
A number of prior art approaches have been taken to quantum well intermixing. U.S. Pat. No. 5,395,793 discloses a method whereby a low energy implant is used to produce a distribution of defects 20 in a layer 3 above the quantum well heterostructure layers 6 to 18, as shown in FIG. 1. Following implantation, the heterostructure 2 is heated, and some of the defects diffuse down through the heterostructure 2, intermixing the wells.
However, because the well regions are not directly irradiated, the defects must diffuse there from the implanted regions, which can result in (i) a non-uniform distribution of defects within the heterostructure 2, and (ii) poor efficiency, since the defects diffuse in all directions away from the implanted regions, and many will be trapped or annihilated at nearby interfaces. Moreover, only low ion fluences are used in order to avoid the formation of stable defect complexes and extended defects, which would trap many of the point defects formed during the implant. This is especially true if an amorphous layer is formed, which greatly reduces the relative number of point defects diffusing into the heterostructure 2, and leads to the formation of stable ‘end of range’ dislocation loops. The presence of these stable defects is likely to degrade the performance of optoelectronic devices fabricated from the heterostructure 2. Consequently, the amount of intermixing available from a single implant is limited, and multiple interleaved implant and annealing sequences are required to obtain substantial layer intermixing and wavelength shifts.
FIG. 2 shows a second approach used in the prior art, whereby a single implant is made directly into the heterostructure 2 to create a distribution of defects 22 within the heterostructure 2. This increases the efficiency of the process, since the defects 22 are created within the heterostructure 2. Hence all of the diffusing point defects are available for intermixing. However, because the defect depth distribution 22 varies considerably over the heterostructure 2, the degree of intermixing also varies considerably, with layers 11, 12, and 18 remaining largely unaffected by the implant. Hence the optical wavelength range of the heterostructure may be undesirably broadened. This disadvantage can be alleviated by performing multiple implants with different ion energies and fluences, calculated to give an approximately uniform distribution across the entire heterostructure 2. FIG. 3 shows a number of defect distributions 30 directed into the heterostructure 2, with a cumulative defect distribution 32 which is nearly constant within the heterostructure boundaries 5 and 19. In such a case, the heterostructure 2 may be annealed after each implantation step, or after all of the implantation steps have been completed. This multiple implant technique gives generally satisfactory results, but requires mathematical simulation of the net defect distribution 32, and is time consuming and expensive due to the multiple implant (and possibly anneal) steps.
It is desired, therefore, to provide one or more of an improved method for disordering a quantum well heterostructure, a quantum well heterostructure disordered by an improved method, an optoelectronic device including such a heterostructure, and an improved method for tuning the wavelength range of an optoelectronic device, or at least useful alternatives.